Optical networks form the backbone of today's telecom and datacom networks. Optical networks exploit the high-transmission capacity of optical fibers by transmitting data over multiple wavelengths simultaneously, similarly to FM radio channels that are transmitted over several frequencies in the 88 to 108 MHz range. The optical frequencies (or wavelengths) in these networks, which are known as Wavelength Division Multiplexed (WDM) networks, are spaced equally (e.g., by 50 Ghz, or by 100 Ghz, etc.) in the frequency domain. The frequencies themselves (in the 195 THz range) are defined by standards set by the International Telecommunications Union (ITU).
Optical networks can be classified broadly into 3 categories: 1) long-haul networks that exist between cities or continents, 2) metro networks that exist within a city, and 3) access networks that typically provide service to residential or business customers. FIG. 1 illustrates a point-to-point link over a long-haul WDM network in which multiple wavelength channels are multiplexed (MUX) together and transmitted over an optical fiber. At the receiving end, the signal is demultiplexed (deMUX) to its constituent wavelengths. These MUX/deMUX operations are performed by optical filters such as Bragg gratings, Thin-film filters (TFF) and AWGs. Over the last few years, as the number of channels transmitted has increased from 4 to 40, AWGs have become the preferred option for filter devices. In recent years, even in the metro network area, there has been a lot of activity in providing increased capacity by increasing the number of channels that are part of a metro ring.
However, in the metro market, there is also the need to dynamically provide bandwidth while maintaining low costs. Setting up and tearing down high-capacity optical wavelength connections for business service transport has been hamstrung by the exorbitant costs of manual configuration. Hence, low-cost reconfigurable optical add/drop multiplexing (ROADM) products are now being introduced for the Metro market. ROADMs give network administrators the ability to select, via software, which of the WDM channels to add, drop or pass-through at each site in a WDM network, thus allowing seamless addition of services as end-user demand necessitates.
FIG. 2 illustrates a ROADM architecture that shares three functional elements: a demultiplexer 10, a switching element 12, and a multiplexer 14. The switching element 12 selects which wavelength can be added or dropped at any given port. AWGs may be used for the multiplexer and demultiplxer operations in this ROADM architecture.
FIG. 3 illustrates a typical AWG 20 that is currently available. AWGs, sometimes referred to as “PHASARS,” are well known components in the optical communications industry. An AWG is an integrated optics planar waveguide structure that acts like a bulk diffraction grating in a spectrometer. The construction and operation of such AWGs are well known in the art. The embodiment of FIG. 3 consists of an array of channel waveguides 30 that connect an input Free Propagation Region (FPR) 32 to the output FPR 34. There are also sets of input waveguides 18, which carry the light to the input FPR 32, and another set of output waveguides 42 that carry the light out from the output FPR 34. The FPR is sometimes referred to as a slab waveguide. In an FPR, an optical beam is confined in only one direction (vertical) as opposed to two directions as in a channel waveguide.
The arrayed waveguide grating region 30 consists of channel waveguides that are of varying length. The waveguides vary in length by increments of ΔL such that, if there were seven waveguides, their lengths would be x+3ΔL, x+2ΔL, x+ΔL, x, x−ΔL, x−2ΔL, and x−3ΔL. Different wavelengths traveling through the array experience different amounts of time delay. The interference and diffraction caused by the different amounts of delay in each waveguide causes the radiation components having different wavelengths to emerge at different angles from the output end of the array waveguide grating region 30.
When used as a demultiplexer, the signal enters through one of the input waveguides 18 carrying all the wavelength signals, which separate after passing through the arrayed waveguide grating region 30 and the output FPR 34. The output waveguides 42 are placed at an are known as the grating circle or the Rowland circle, where the constituent wavelengths of the signal focus separately such that each output waveguide carries a separate signal wavelength.
When used as a multiplexer, all the separate signal wavelengths enter the device through multiple input waveguides 18, pass through the arrayed waveguide grating region 30 and focus at the same Rowland circle point in the output FPR 34. The output waveguide 42 placed at this focus point then carries the signal away to couple to an optical fiber (not shown) with all the wavelength signals multiplexed together.
In theory, a single AWG 20 can be used both as a Mux as well as a De-Mux due to its bi-directional nature. However, in practice, a single AWG 20 is never used to perform both functions simultaneously because optical communication system vendors have very different requirements for the performance of AWG filters when used as a MUX and when used as a deMUX. A spectral filter is characterized by its spectral passband shape, as well as other parameters such as loss and crosstalk to other channels.
The AWG 20, when used to de-multiplex wavelengths that are 100 GHz apart (˜0.8 nm), requires a passband spectral width of 0.2 nm 1 dB down from the peak transmission and a Full width at Half Maximum (FWHM-or 3 dB down from peak transmission) of 0.4 nm. Crosstalk characteristics are also important and typical numbers for adjacent channel crosstalk require the power of a signal in its neighboring channels to be 25 dB below the peak transmission in the channel. This is known in the art as a Gaussian-shaped spectral passband.
FIGS. 4A and 4B show exemplary spectral passband shapes of the filter through which the deMUX and MUX signals pass, respectively. In both figures, the x-axis indicates the wavelength in nm and the y-axis indicates the loss in dB. Generally, it is desirable for the AWG deMUX spectral passband filter to have a flat-topped passband shape with sharper slopes than what a Gaussian passband would offer. This is because a Gaussian passband requires tight control over drift of the wavelengths emitted from the lasers, making it difficult to use in transmission systems. As shown, the deMUX requirements for 1 dB and 3 dB passbands are 0.4 nm and 0.6 nm, respectively, along with an adjacent channel crosstalk of 25 dB. This is known in the art as a flat-top passband.
In contrast, the MUX requirements for 1 dB and 3 dB passband are 0.5 nm and 0.8 nm, respectively, while there is no crosstalk specification (as the signals are mixed together anyway). As shown, the passband shape for a MUX device is wide-band Gaussian. Due to the different passband shapes, two substantially different AWG designs have to be used for the Mux and Demux devices.
FIGS. 5A and 5B illustrate a MUX/deMUX device in separate packages as a deMUX package 50 and a MUX package 52, each package having a separate AWG unit. Essentially, any network system requiring a MUX and a deMUX would require two separate AWG units, each specifically designed to the different specifications of passband shape and packaged separately even though they would operate over the same wavelengths. Each packaged device 50, 52 typically consists of the AWG circuit bonded to fiber arrays which couple the light into and out of the circuit, a heater/TEC 54 and a temperature sensor 56 like an RTD or thermistor. The AWG is a temperature-sensitive device inasmuch as the passband wavelengths can drift with ambient temperature. Hence, the AWG is temperature-controlled (external to the package) by a controller, which is typically in a system card. The system card reads the sensor's temperature and adjusts the heater/TEC 54 accordingly. Each of the packaged devices 50, 52 includes the arrayed waveguide grating region 30 between an input FPR 32 and an output FPR 34. Since MUX and deMUX are in separate packages that may or may not have different temperature set points, the card that controls each of the AWG packages 50, 52 include two separate controllers and spaces for two AWG packages. This substantially increases the footprint of the network system and also affects cost and complexity.
FIGS. 6A and 6B illustrate different embodiments of a MUX/deMUX device 60 wherein the MUX and deMUX circuits are housed in the same AWG package. In this case, the AWG temperatures may have to be controlled separately as they may have different set points. In some cases, the network system customer may require the AWG package to have internally-built temperature controllers that require no external control logic in the system card. In such a case, the packaging becomes more complicated and the package footprint grows larger due to the need for two controller circuits.
Overall, both the separately-packaged device of FIGS. 5A and 5B and the single-package embodiment of FIG. 6 have the problem of needing two different AWGs with different spectral characteristics for MUX and DeMUX while using the same operating wavelengths. Also, both embodiments pose packaging complexities of controlling the different set-point temperature of the two devices/circuits, leading to larger footprints and higher costs.